Direct measurement of the flow rate of steel melt near the surface thereof in a crystallizer allows control over the flow field of the steel melt in the crystallizer, so as to reduce effectively the surface flaws of a continuous casting blank caused by entrapped mold flux, inclusions, bubbles and the like, which, in turn, reduces the occurrence of the surface flaws of a cold rolled plate such as an automobile shell plate, silicon steel, etc. Apparently, the key of the control over the flow rate near the surface in a crystallizer is measurement of the flow rate of the steel melt near the surface in the crystallizer.
The flow field of the steel melt in a crystallizer or tundish is of great significance to the control over inclusions and the surface quality of a casting blank in the process of steel making. Due to the high temperature up to about 1600° C. of steel melt, the devices and methods commonly used for measuring liquid flow rates do not work. Hence, measurement of the flow rate of high-temperature steel melt becomes a difficult technical problem in the field of steel making.
In a continuous casting process, the mold flux in a crystallizer functions to prevent oxidation of the steel melt surface, act as a lubricant between the crystallizer and a casting blank, capture floating inclusions, and keep the temperature of the steel melt. However, in order to improve the surface quality of a continuous casting blank, inclusion of the mold flux in the crystallizer into the steel melt must be inhibited.
As shown in FIGS. 1 and 2, in a continuous casting process of steel making, the steel melt in a tundish 10 is infused into a crystallizer 5 through a tundish dam 9, a slide nozzle 11 and a submerged nozzle 1 in sequence. The steel melt flows out from the outlet 3 of the submerged nozzle, impinges on the short-side wall of the crystallizer, and forms an upward countercurrent 4 moving toward the steel melt surface in the crystallizer and a downward countercurrent 6 moving toward the bottom of the crystallizer.
When the upward countercurrent 4 exhibits an unduly large flow rate, the fluctuation of the steel melt surface in the crystallizer tends to be increased, resulting in engulfment of the mold flux into the steel melt. If the engulfment and mixing of the mold flux occurs, surface flaws of the cold rolled steel plate will be incurred, and thus the product yield will be decreased. However, when the upward countercurrent 4 exhibits an unduly small flow rate, the flowability of the steel melt adjacent to the meniscus will be decreased. As a result, the temperature of the steel melt at the meniscus will decrease, and thus the mold flux will not be melted sufficiently, such that the function of the mold flux for capturing floating inclusions will be degraded. Instead, the inclusions and mold flux are captured at the solidified shell 7 near the meniscus, which also leads to increased occurrence of flaws in the final product of cold rolled strip.
Therefore, development of a suitable flow field pattern and a suitable flow rate distribution in the crystallizer is critically important for the control of inclusions in a continuous casting blank and the surface quality of the continuous casting blank, as well as the surface quality of a cold rolled product such as an automobile shell plate, etc.
It is proposed in a patent literature (Japanese Patent Publication H 4-178525) that a ceramic rod is inserted into steel melt and applies a pressure to a pressure sensor arranged above the ceramic rod along the flow direction due to the impact of the flowing movement of the steel melt. The pressure sensed by the pressure sensor may be converted into the flow rate near the surface of the steel melt. Unfortunately, the device used in this measurement method is rather complicated. Moreover, the harsh high-temperature environment also influences the precision and stability of the pressure sensor in operation, leading to large error in flow rate measurement. In addition, the pivot of the pressure sensing rod for flow rate measurement is located adjacent to the upper top of the sensing rod. As such, a large moment is needed to rotate the sensing rod to a particular angle. Hence, the sensitivity of the flow rate measurement is not good. The influence is even more remarkable when the flow rate of the steel melt in a crystallizer and the like is low.